RAM (DRAM) is used at run time for executable Cisco IOS software (and its subsystems), routing tables, Fast Switching cache, running configuration, packets, and so on.
Non-Volatile RAM (NVRAM) is used for writable permanent storage of the startup configuration. It is an EPROM, where the startup configuration is stored in the same Flash device where the boot code is loaded.
BootROM is used for permanently storing startup diagnostic code (ROM Monitor) and emergency OS
The main task for the BootROM is to perform some hardware diagnostics during bootup on the router (Power On Self Test - POST), and to load the Cisco IOS software from the Flash to the Memory.
The BootROM is not erasable; it is socketed so it can be replaced.
Flash is used for permanent storage of a full Cisco IOS software image in compressed form.